Wafer center finding

ABSTRACT

A number of wafer center finding methods and systems are disclosed herein that improve upon existing techniques used in semiconductor manufacturing.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.11/682,306 filed on Mar. 5, 2007.

The '306 application is a continuation-in-part of U.S. application Ser.No. 11/679,829 filed on Feb. 27, 2007, which claims the benefit of U.S.Prov. App. No. 60/777,443 filed on Feb. 27, 2006, and is acontinuation-in-part of U.S. application Ser. No. 10/985,834 filed onNov. 10, 2004 which claims the benefit of U.S. Prov. App. No. 60/518,823filed on Nov. 10, 2003 and U.S. Prov. App. No. 60/607,649 filed on Sep.7, 2004.

The '306 application also claims the benefit of the following U.S.applications: U.S. Prov. App. No. 60/779,684 filed on Mar. 5, 2006; U.S.Prov. App. No. 60/779,707 filed on Mar. 5, 2006; U.S. Prov. App. No.60/779,478 filed on Mar. 5, 2006; U.S. Prov. App. No. 60/779,463 filedon Mar. 5, 2006; U.S. Prov. App. No. 60/779,609 filed on Mar. 5, 2006;U.S. Prov. App. No. 60/784,832 filed on Mar. 21, 2006; U.S. Prov. App.No. 60/746,163 filed on May 1, 2006; U.S. Prov. App. No. 60/807,189filed on Jul. 12, 2006; and U.S. Prov. App. No. 60/823,454 filed on Aug.24, 2006.

All of the foregoing applications are commonly owned, and all of theforegoing applications are incorporated herein by reference in theirentirety.

BACKGROUND

In semiconductor manufacturing, wafers and other substrates aretransferred among various process chambers using robotic handlers. Oneof the enduring challenges of wafer handling is the need to locatewafers or wafer centers with sufficient precision to permit accurateplacement and processing within the process chambers. In general,semiconductor manufacturing systems employ various beam-breaking sensorarrangements to “stripe” passing wafers and detect the wafer edges. Thisdata can be used, in turn to locate a wafer center relative to a robotichandler so that subsequent movement and placement can be more accuratelycontrolled. Center finding is sufficiently important to fabrication thatthis process is routinely calibrated and repeated throughout theprocessing of each wafer.

While numerous physical sensors and processing algorithms have beendevised for centering wafers in a semiconductor manufacturing process,there remains a need for improved wafer center finding techniques thatreduce the number of sensors required or improve the simplicity and/oraccuracy of center finding calculations.

SUMMARY

A number of wafer center finding methods and systems are disclosedherein that improve upon existing techniques used in semiconductormanufacturing.

In one aspect, a method for finding a center of a wafer in a devicehaving an interior and a plurality of entrances, the interior containinga robotic arm, and the device including a plurality of sensors, each oneof the sensors adapted to detect a presence of the wafer at apredetermined location within the interior of the device, includes:retrieving the wafer from outside the interior through a first one ofthe plurality of entrances; retracting the wafer into the interior anddetecting a presence of the wafer with a first one of the plurality ofsensors; rotating the robotic arm; extending the wafer out of theinterior through a second one of the plurality of entrances anddetecting an absence of the wafer with the first one of the plurality ofsensors; and determining a location of a center of the wafer relative tothe robotic arm using sensor data from the plurality of sensors andposition data from the robotic arm.

The plurality of sensors may include optical sensors. The plurality ofsensors may include light emitting diodes. The plurality of sensors mayinclude auto focusing photodiode detectors. Determining a location mayinclude applying a Kalman Filter to the position data from the roboticarm. The method may include updating the Kalman Filter based upon thesensor data. The wafer may be substantially circular. The wafer mayinclude an alignment notch. The plurality of sensors may include atleast one detector positioned opposite a light emitting diode such thata light path from the light emitting diode to the light detectorincludes a predetermined position within the interior. The plurality ofsensors may include at least one detector positioned such that lightfrom a light emitting diode, when reflected off of the wafer at apredetermined location, is detected by the detector. Retracting mayinclude may include retracting in a linear motion. Extending may includeextending in a linear motion. Rotating may include rotating about acenter axis of the robotic arm.

In another aspect, a method disclosed herein for finding a center of awafer in a device having an interior and a plurality of entrances, theinterior containing a robotic arm, and the device including a pluralityof sensors, each one of the sensors adapted to detect a presence of thewafer at a predetermined location within the interior of the device,includes: retrieving the wafer from outside the interior through a firstone of the plurality of entrances; retracting the wafer into theinterior; rotating the robotic arm; extending the wafer out of theinterior through a second one of the plurality of entrances; detectingthe presence of the wafer at a predetermined location of at least onesensor during the retracting, rotating, and extending, thereby providingsensor data; and determining a location of a center of the waferrelative to the robotic arm using the sensor data and position data fromthe robotic arm.

The plurality of sensors may include optical sensors. The plurality ofsensors may include light emitting diodes. The plurality of sensors mayinclude auto focusing photodiode detectors. Determining a location mayinclude applying a Kalman Filter to the position data from the roboticarm. The method may include updating the Kalman Filter based upon thesensor data. The wafer may be substantially circular. The wafer mayinclude an alignment notch. The plurality of sensors may include atleast one detector positioned opposite a light emitting diode such thata light path from the light emitting diode to the light detectorincludes a predetermined position within the interior. The plurality ofsensors may include at least one detector positioned such that lightfrom a light emitting diode, when reflected off of the wafer at apredetermined location, is detected by the detector. Retracting mayinclude retracting in a linear motion. Extending may include extendingin a linear motion. Rotating about a center axis of the robotic arm.Detecting the presence of the wafer may include detecting a firsttransition from absence to presence of the wafer at one of the pluralityof sensors and detecting a second transition from presence to absence ofthe wafer at the one of the plurality of sensors, wherein a path of thewafer from the first transition to the second transition is non-linear.The path may include at least one arc resulting from a rotation of thewafer.

In another aspect, a device for handling a wafer disclosed hereinincludes: an interior accessible through a plurality of entrances; and aplurality of sensors consisting of two sensors for each one of theplurality of entrances, each sensor capable of detecting a presence of awafer at a predetermined location within the interior, wherein theplurality of sensors are arranged so that at least two of the pluralityof sensors detect the wafer for any position of the wafer entirelywithin the interior.

The plurality of entrances may include four entrances. The plurality ofentrances may include seven entrances. The plurality of entrances mayinclude eight entrances. The plurality of sensors may include opticalsensors. The plurality of sensors may include at least one lightemitting diode. The device may include a robotic arm having a centeraxis within the interior, the robotic arm including an end effector forhandling wafers.

In another aspect a device for handling a wafer disclosed hereinincludes: an interior accessible through a plurality of entrances; and aplurality of sensors consisting of two sensors for each one of theplurality of entrances, each sensor capable of detecting a presence of awafer at a predetermined location within the interior, wherein theplurality of sensors are arranged so that a first pair of sensors detecta wafer entering linearly through each one of the plurality of entrancesand a second pair of sensors are positioned substantially immediatelyoutside the maximum diameter of the wafer entering linearly through eachone of the plurality of entrances, and wherein each one of the pluralityof entrances shares one of the first pair of sensors and the second pairof sensors with each neighboring one of the plurality of entrances.

The plurality of entrances may include four entrances. The plurality ofentrances may include seven entrances. The plurality of entrances mayinclude eight entrances. The plurality of sensors may include opticalsensors. The plurality of sensors may include at least one lightemitting diode. The device may include a robotic arm having a centeraxis within the interior, the robotic arm including an end effector forhandling wafers.

In another aspect, a device for handling a wafer disclosed hereinincludes: an interior accessible through four entrances; and eightsensors, each capable of detecting a presence of a wafer at apredetermined location within the interior, the sensors arranged intotwo square arrays centered about a center of the interior, sized suchthat a first one of the square arrays is smaller than a second one ofthe square arrays and oriented so that a group of four of the sensors atopposing vertices of the two square arrays are collinear.

The eight sensors may include optical sensors. The eight sensors mayinclude at least one light emitting diode. The device may include arobotic arm having a center axis within the interior, the robotic armincluding an end effector for handling wafers.

In another aspect, a device disclosed herein may include a robotic armfor handling a wafer, the robotic arm including one or more encodersthat provide encoder data identifying a position of one or morecomponents of the robotic arm; and a processor adapted to apply anextended Kalman Filter to the encoder data to estimate a position of thewafer.

The position may include a wafer center and/or a wafer radius. Theposition may be determined with reference to an end effector of therobotic arm. The position may be determined with reference to a centeraxis of the robotic arm. The processor may recalculate the position eachtime new encoder data is received. The new encoder data may be receivedat substantially 2 kHz. The processor may be adapted to update one ormore equations of the Kalman Filter using transition data from one ormore sensors that detect the presence of a wafer at one or morepredetermined locations within a robotic wafer handler.

In another aspect, a method disclosed herein includes disposing aplurality of sensors within an interior of a wafer handling device, eachone of the plurality of sensors capable of detecting a transitionbetween presence and absence of a wafer at a predetermined locationwithin the interior; handling a wafer with a robotic arm, the roboticarm including one or more encoders that provide encoder data identifyinga position of one or more components of the robotic arm; and applyingthe encoder data to an extended Kalman Filter to provide an estimatedposition of the wafer.

The method may include detecting a transition at one of the plurality ofsensor to provide an actual position of the wafer; determining an errorbetween the actual position and the estimated position; and updating oneor more variables for the extended Kalman Filter based upon the error.Applying the encoder data may include calculating a wafer position every0.5 milliseconds. The estimated position of the wafer may include acenter of the wafer. The estimated position of the wafer may include aradius of the wafer.

In another aspect, a device disclosed herein may include an interiorchamber having a plurality of entrances shaped and sized for passage ofat least one wafer; a contact image sensor positioned to scan a waferwithin the interior; a robot within the interior including an endeffector for handling the wafer, the robot configured to move the waferwithin a measurement volume of the contact image sensor therebyobtaining an image of at least a portion of the wafer; and a processorconfigured to process the image and determine a center of the wafer.

The robot may move the wafer linearly through the measurement volume ofthe contact image sensor. The contact image sensor may be orientednormal to a path of the wafer. The contact image sensor may be orientedat a forty-five degree angle to a path of the wafer. The robot may movethe wafer in a curved path through the measurement volume of the contactimage sensor. The robot may move the wafer in a discontinuous paththrough the measurement volume of the contact image sensor. The robotmay rotate the wafer within the measurement volume of the contact imagesensor. The robot may be adapted to lift the wafer into the measurementvolume of the contact image sensor. The robot may include a rotatingchuck on an end effector adapted to rotate the wafer. The rotating chuckmay rotate between one-hundred eighty degrees and three-hundred sixtydegrees. The device may include a rotating chuck adapted to lift thewafer from the end effector into the measurement volume of the contactimage sensor. The contact image sensor may be at least 300 mm in length.The contact image sensor may have a length exceeding a diameter of thewafer. The contact image sensor may be positioned at one of theplurality of entrances to the interior. The device may include aplurality of contact image sensors, each one of the plurality of contactimage sensors place at one of the plurality of entrances to theinterior. The contact image sensor may be placed to intersect a centerof the interior. The device may include a second contact image sensor,wherein the contact image sensor and the second contact image sensor arepositioned collinearly. The contact image sensor and the second contactimage sensor may be positioned at one of the plurality of entrances tothe interior. The device may include a plurality of pairs of collinearcontact image sensors positioned at each one of the plurality ofentrances to the interior. The device may include a second pair ofcollinear contact image sensors, wherein the second pair of collinearcontact image sensors are positioned to intersect a center of theinterior. The plurality of entrances may include four entrances. Theplurality of entrances may include eight entrances. The processor may befurther configured to identify an alignment notch on the wafer. Theprocessor may be further configured to determine a radius of the wafer.

In another aspect, a method may include positioning a contact imagesensor to capture image data from the interior of a robotic waferhandler; passing at least a portion of a wafer by the contact imagesensor to acquire an image; and determining a center of the wafer basedupon the image. Passing at least a portion of the wafer by the contactimage sensor may include passing the wafer linearly through ameasurement volume of the contact image sensor.

In another aspect, a device disclosed herein includes a robotic armwithin a robot chamber, the robotic arm including an end effectoradapted to handle a wafer; and a linear array of charge-coupled deviceswithin the interior of the robot chamber, the linear array positioned toacquire image data from a measurement volume in one or morepredetermined locations within the robot chamber.

The device may include an external illumination source that illuminatesthe linear array. The device may include a wireless power coupling thatinductively powers the linear array. The device may include a wirelesstransceiver for exchanging data wirelessly with the linear array. Thewireless transceiver may be positioned outside the robotic chamber. Thedata may include the image data. The linear array may be a 1 by n arrayof charge-coupled devices. The linear array may include a twodimensional array of charge-coupled devices. The device may include aplurality of linear arrays each capturing image data at a differentlocation within the interior. The robotic arm may include a chuck on theend effector adapted to rotate the wafer within the measurement volumeof the linear array. The robotic arm may be adapted to lift the waferinto the measurement volume of the linear array. The chuck may rotatebetween one-hundred eighty degrees and three-hundred sixty degrees. Thedevice may include a rotating chuck adapted to lift the wafer from theend effector into the measurement volume of the linear array. The devicemay include a processor configured to determine a center of the waferusing the image data. The device may include a processor configured todetermine a radius of the wafer using the image data. The device mayinclude a processor configured to identify an alignment notch on thewafer using the image data.

In another aspect, a device disclosed herein may include a robotic armwithin a robot chamber, the robotic arm including an end effectoradapted to handle a wafer; and a linear array of charge-coupled deviceson the end effector positioned to capture edge data from a wafer restingon the end effector.

The device may include an external illumination source that illuminatesthe linear array. The device may include a wireless power coupling thatinductively powers the linear array. The device may include a wirelesstransceiver for exchanging data wirelessly with the linear array. Thewireless transceiver may be positioned outside the robotic chamber. Thelinear array may be a 1 by n array of charge-coupled devices. The lineararray may include a two dimensional array of charge-coupled devices. Therobotic arm may include a chuck on the end effector adapted to rotatethe wafer within the measurement volume of the linear array. The devicemay include a rotating chuck adapted to lift the wafer from the endeffector and rotate the wafer within the measurement volume of thelinear array. The device may include a processor configured to determinea center of the wafer using the edge data. The device may include aprocessor configured to determine a radius of the wafer using the edgedata. The device may include a plurality of linear arrays positioned tocapture edge data from a number of locations on a surface of the endeffector.

These and other systems, methods, objects, features, and advantages ofthe present invention will be apparent to those skilled in the art fromthe following detailed description of the preferred embodiment and thedrawings. All documents mentioned herein are hereby incorporated intheir entirety by reference.

BRIEF DESCRIPTION OF THE FIGURES

The invention and the following detailed description of certainembodiments thereof may be understood by reference to the followingfigures:

FIG. 1 shows a top view of a wafer handling module with eight sensorsfor detecting the position of a wafer.

FIG. 2 shows a top view of a wafer handling module with four sensors fordetecting the position of a wafer.

FIG. 3 illustrates a generalized process for wafer center finding.

FIG. 4 illustrates a wafer center finding process using a Kalman Filter.

FIG. 5 shows a device with a linear image sensor.

FIG. 6 shows a top view of a contact image sensor used for wafer centerfinding with a linear wafer motion.

FIG. 7 shows a top view of a contact image sensor used for wafer centerfinding with a curving wafer motion.

FIG. 8 shows a top view of a contact image sensor used for wafer centerfinding with a rotating wafer motion.

FIG. 9 shows a pair of linear CCD arrays used for wafer center findingwith a linear wafer motion.

FIG. 10 shows a single CCD array used for wafer center finding with arotating wafer motion.

FIG. 11 shows four CCD arrays used for wafer center finding with acompound wafer motion.

FIG. 12 shows CCD sensors on a robotic arm end effector.

FIG. 13 shows a single CCD sensor on an end effector with a rotatingchuck.

FIG. 14 shows a single CCD in a robotic handling module.

DETAILED DESCRIPTION

The following description focuses on finding the center of substantiallycircular semiconductor substrates with alignment notches. However, itwill be understood that suitable adaptations may be made to many of thetechniques described below for finding centers of other geometries suchas ovals, half-circles, squares, rectangles, and so forth. It willfurther be understood that, while semiconductor fabrication is animportant field for use of the techniques described herein, that thecenter finding techniques discussed below have broad applicability, andmay be used for example in a wide range of machine vision and roboticsapplications.

The term “wafer” as used herein is a short hand for all substrates andother materials that might be handled by a semiconductor fabricationsystem. It will be understood that, while the following description isapplicable to wafers, and refers specifically to wafers in a number ofillustrative embodiments, a variety of other objects may be handledwithin a semiconductor facility including a production wafer, a testwafer, a cleaning wafer, a calibration wafer, or the like, as well asother substrates (such as for reticles, magnetic heads, flat panels, andthe like), including substrates having various shapes such as square orrectangular substrates. All such workpieces are intended to fall withinthe scope of the term “wafer” as used herein unless a different meaningis explicitly provided or otherwise clear from the context.

FIG. 1 shows a top view of a transfer robot handling module. In themodule 110, a substantially round wafer 120 may be handled by a robot(not shown) having a center axis 160 while sensors detect the presence(or absence) of the wafer 120. In general, the module 110 may have asubstantially circular interior 170 with a radius sufficient toaccommodate rotational movement of a wafer and the robot among variousentrances (not shown) to the module 110. While additional void space maybe provided, and the shape may have any geometry that can accommodatemovement of wafers, the generally circular shape provides a significantadvantage of minimizing volume within the vacuum environment maintainedby the module 110 and other related hardware.

Also in general, two or more entrances may be provided to the module 110shaped and sized for passage of the wafer 120, along with any portionsof the robotic arm required to place and retrieve the wafer 120 outsidethe module 110. In general the size of each entrance would be only wideenough and tall enough to accommodate a single wafer, along with an endeffector and any other portions of the robot that must pass through theentrance during handling. This size may be optimized by having the robotmove wafers straight through a center of each entry, whichadvantageously conserves valuable volume within the vacuum environment.Semiconductor wafers are generally substantially circular as provided byindustry standards. Such wafers may also include an alignment notch forpreserving a rotational alignment of the wafers during processing, andidentifying or accounting for this notch may require additionalprocessing during wafer center finding, as described in greater detailbelow. However, more generally a wafer may have a variety of shapesand/or sizes. For example, while 300 mm is a conventional size forcurrent wafers, new standards for semiconductor manufacturing providefor wafers over 400 mm in size. Further, certain substrates have othershapes, such as rectangular substrates employed for flat panels. Thus itwill be understood that the shape and size of components (and voids)designed for wafer handling may vary, and one skilled in the art wouldunderstand how to adapt components such as the entrances to particularwafer dimensions.

In one embodiment, the module 110 may include four entrances, one oneach side of the module 100. The module 110 may also include a differentnumber of entrances such as two or three. Further, while a square module110 is depicted, the module 110 may have other shapes (such as commonlyused in cluster processing), such as a rectangle or a regular polygonsuch as a hexagon, heptagon, octagon, or the like. A rectangular shapemay have multiple entrances on one side, and the regular polygons mayinclude an entrance on each side. Thus it will be understood that whilea square module 110 having an entrance on each side is a commonarrangement useful for semiconductor manufacturing, many other shapesmay be suitably adapted for use in a fabrication facility and areintended to fall within the scope of this disclosure.

As depicted, the sensors may include eight sensors 131-138 arranged astwo square arrays 141, 142 centered about the center axis 160 of therobot. The sensors are arranged to that four of the sensors 131-134 forma first, inner array 142 while four of the sensors 135-138 form asecond, outer array 141. While the layout of these sensors is bestunderstood with reference to FIG. 1, other features of the layout aredescribed as follows. The two concentric square arrays 141, 142 areoriented so that the vertices form pairs 150 of sensors from the innerarray 142 and outer array 141. The arrays 141, 142 are furtherrotationally oriented so that four such sensor from opposing vertices ofthe two square arrays 141, 142 are collinear, forming a line thatintersects a center of the interior 170 or the center axis 160 of therobot. This last constraint is not strictly necessary—the robot may havemore than one axis, and the robot may be adapted for a variety ofrotational movements that would not require an axis at the center of theinterior 170. However, it is a convenient and practical layout for arobotic handler that provides three-hundred and sixty degree freedom ofmotion. It will also be noted that when the wafer 120 first enters (orexits) the interior 170 from one of the entrances, which are generallycentered on each side, two sensors from the inner array 142 can detectthe wafer, and two sensors from the outer array 141 are positionedimmediately outside a diameter of the wafer 120 on either side. In thismanner, it can be ensured that, while maintaining a ratio of only twosensors for each entrance, at least two sensors detect the wafer 120 atall times while the wafer 120 is within the interior 170, and at leastone sensor will immediately detect any rotational movement of the wafer120 within the interior 170. As a significant advantage, thisconfiguration also ensures that it is always possible to detect thepresence of a wafer within the interior, even if the module 110 andsensors 131-138 are powered up, for example after a power failure, withno a priori data concerning wafer position.

A similar arrangement may be provided for a module having five, six,seven, eight, or more entrances. In general, each entrance may have twosensor on each side, where a first sensor is positioned to detect awafer when it has been moved entirely into the interior from theentrance and a second sensor is positioned immediately outside thediameter of the wafer. In such embodiments, each pair of sensors from aninner and outer array may be shared with a neighboring entrance, thatis, an immediately adjacent entrance on either side thereof.

While FIG. 1 depicts a specific arrangement of sensors 131-138, othercriteria may be used to determine suitable sensor numbers and placement.For example, sensor placement may advantageously provide at least fourpoints around the circumference of the wafer during any move sequencewhere the wafer is retrieved from a station and placed into anotherstation. Any group of three points used to estimate the center andradius may usefully contain more than sixty degrees between the at leastthree points, less than one-hundred eighty degrees between any threepoints used to define a center and radius (i.e., no section ofone-hundred sixty degrees should lack a point defining its edge. Extrapoints may advantageously be employed to improve an estimate throughdirect calculation or to validate a calculate circle. Sensors mayadvantageously be positioned within the swing radius of links of arobotic arm, along with a fiducial marking that can reliably andrepeatably trigger the sensors.

Sensor arrangements may also be adapted to specific end effectors. Forexample, fork-type end effectors support a wafer around the side edges,but not the front. For conventional wafer sizes, this leaves a 250 mmwide area in the middle of a fork; however, none of the side edge may beused for detection. For a paddle-type end effector, a center 150 mm,straddling a center line of linear extension is open for sensorpositioning, but the back end of the wafer, toward the wrist of therobotic arm, may be completely blocked from the sensor by theend-effector.

The sensors 131-138 generally operate to detect the presence of a waferat a predetermined position within the interior 170. It will beunderstood that, as used herein, detecting a presence includes detectingan absence as well as detecting a transition between absence andpresence of a wafer. A number of technologies may be suitably employedfor this type of detection including optical sensors such as reflectivetechnologies where a light source is reflected back toward a source whena wafer is present or beam-breaking technologies where a beam between alight source and a sensor is broken when a wafer is present. In oneembodiment, the sensors 131-138 employ a light-emitting diode or laserlight source with light directed toward an auto focusing photodiodedetector (which facilitates alignment during installation). It will beunderstood that while the sensors described above are one cost effectivesolution for detecting the presence of a wafer at predeterminedlocations, other sensing technologies may be similarly employed providedthey can be adapted to vacuum semiconductor environments. This mayinclude, for example, sonar, radar, or any other electromagnetic orother distance or position sensing technology.

The distance between the inner array 142 and the outer array 141, oreach pair of sensors 150 therein, will generally be determined by thesize of wafers handled by the system. In one embodiment, the positionsof sensors may be adjustable to form larger or smaller arrays whilemaintaining the linear and diagonal relationships discussed above. Inthis manner, the module 110 may be readily adapted to wafers ofdifferent sizes.

In general operation, the sensors 131-138 are employed to determine alocation of a center of the wafer 120, using circular models, linearmodels (such as the Kalman Filter technique described below), or anyother suitable mathematical, neural network, heuristic, or othertechnique. Methods for detecting wafer position and center are nowdescribed in greater detail. In general, the following techniques employa combination of data from the sensors 131-138 and data from encodersfor one or more robotic handlers that provide data concerning a positionof robotic components. While the following description focuses on sensorand encoder data, it will be understood that time, as detected by anyclock or signal within the system, may also be used explicitly orimplicitly in wafer center finding calculations.

FIG. 2 shows a top view of a wafer handling module with four sensors fordetecting the position of a wafer. In this embodiment, the system 200may employ only one sensor 202 for each entrance. The sensors 202 may beany of the sensors described above. In this case, the sensors 202 arepreferably positioned near each entrance and inside the diameter of awafer 204 so that at least one edge detection can be obtained as thewafer passes any one of the entrances. As depicted, the wafer handlingmodule 210 is generally square, and includes four entrances, each havingone sensor 202 associated therewith.

FIG. 3 illustrates a generalized process for wafer center finding.

In general, a robotic arm, such as any of the robotic arms describedabove, may engage in a number of operations to transfer a wafer, such asany of the wafers described above, from one location in a semiconductormanufacturing process to another location. This includes a number ofoperations including retrieving a wafer from a first location as shownin step 302, retracting a robotic arm into a module such as any of themodules described above, as shown in step 304, rotating the robotic armtoward another entrance to the module as shown in step 306, extendingthe robotic arm through this entrance as shown in step 308, and placinga wafer in a second location as shown in step 310. The first and secondlocations may be any locations within a fabrication facility includingother robotic handlers, load locks, buffer or transition stations,process modules of any kind, and/or other modules for supplementalprocesses such as cleaning, metrology, scanning, and so forth. Asdepicted in FIG. 3, this process may be repeated indefinitely as wafersare moved in and out of the facility and processed by various processmodules. It will be understood that, while not explicitly depicted,other steps may be performed by the system during these operations, suchas opening or closing isolation valves for entrances to the interior, orwaiting within the interior for access to various resources. The detailsof various robotic handling operations are well known in the art, andany such robotic arms or handling functions may be suitably employedwith the process depicted in FIG. 3. This includes various combinationsof extensions, retractions, and rotations of the robotic arm, z-axismotion by the robotic arm, and any other operations that might beusefully employed in wafer handling.

While the robotic arm is being controlled in a wafer handling operationas described in steps 302, encoders provide data concerning the positionof the robotic arm, either directly or by detecting positions (includingrotational orientation) of drive elements that control operation of therobotic arm. This data may be received for processing as depicted instep 320. As shown in step 330, sensor data may be received from one ormore sensors, such as any of the sensors described above, that detectthe presence, absence, or transition between presence and absence ofwafers at predetermined locations within the robotic handler. It will beunderstood that the physical data for such sensing may come in a varietyof forms including presence of an optical signal, absence of an opticalsignal, strength of an optical signal, or a binary signal encoding anyof the above. All such signals may usefully be employed to senseabsence, presence, and transitions as described herein.

As shown in step 330, the encoder data and the sensor data may beapplied to calculate position data for a wafer such as alignment, wafercenter, and so forth. Details of various algorithms for calculatingwafer position are now provided. While not explicitly shown, it will beunderstood that the controller or other device that calculates waferposition may apply this data in any of a variety of ways to controlfurther movement of the robotic arm. In particular, this data may beused for accurate placement of the wafer at a destination location. Thedata may also be stored, and used as an initial estimate of waferposition when the same wafer is retrieved for an additional movement.

In the four-entrance, four-sensor embodiment of FIG. 2, wafer edge data(obtained as transitions in step 330) is used to determine a wafercenter to a transport path that facilitates moving the wafer from itsdetected position to its destination position. The sensor position,robot position, and destination location positions (such as within aprocess chamber or load lock) are defined in a world coordinate systemthat facilitates determining the relative position of these and otherelements within a wafer processing system that includes the waferhandling robotic module. The world coordinate system may advantageouslybe established with reference to the sensor positions.

Through training, a controller may associate robot positions or encoderdata with the world coordinate system using sensor data to detect, forexample, aspects of the robot end effector and recording concurrentvalues from the encoders. The controller may thus map encoder values toworld coordinates so that as the robot moves, the world coordinateposition of the robot is known. The controller may similarly determinesthe world coordinates of other elements (such as destinations) withinthe wafer processing system to create a world coordinate map of theelements of the wafer processing system. Association of robot positionswith the world coordinate system may also, or instead, be done manually,with a calibrated fixture, or with an instrumented tool carried by therobot. The foregoing is provided by way of example only, and it will beunderstood that many techniques are known in the art for associatedrobotic positions with a world coordinate system and may be usefullyemployed with the systems described herein. For example, while asensor-based world coordinate system is one possible approach, similarcenter finding functions may be performed using an end-effector-basedworld coordinate system.

After the robotic arm has been suitably trained, sensor data may beacquired while a wafer is handled through a retract/rotate/extendmotion, as generally depicted in FIG. 3. A number of techniques may besuitable employed to determine wafer position where a wafer moves in anon-linear path over a number of sensors having predetermined location.Several such techniques are described in detail below by way ofillustration and not of limitation.

To estimate the center and radius of a wafer, the world coordinate edgepoint data may be applied to simultaneous circle equations. Theseequations may be converted to matrix form and a so-called pseudo inversemay be used to provide a least squares solution to the matrix, asdescribed for example in Linear Algebra and its Applications by GilbertStrang (Academic Press, Inc. 1980), the entire content of which isincorporated by reference. This solution minimizes the squared errorbetween a circle's perimeter and the detected edge points. From thissolution, the center location and radius can be calculated. Statedmathematically, the general equation for a circle may be expressed as:

(x−x _(c))²+(y−y _(c))² =r ²

which may be reformulated as:

x ² +y ² +Dx+Ey+F=0

where

D≡−2x _(c) ,E≡−2y _(c) ,F≡x _(c) ² +y _(c) ² −r ²

Given n points from the circumference of this circle, a matrix of nequations may be formed as:

${\begin{bmatrix}x_{1} & y_{1} & 1 \\x_{2} & y_{2} & 1 \\\vdots & \vdots & \vdots \\x_{i} & y_{i} & 1 \\\vdots & \vdots & \vdots \\x_{n} & y_{n} & 1\end{bmatrix}\begin{bmatrix}D \\E \\F\end{bmatrix}} = {- \begin{bmatrix}{x_{1}^{2} + y_{1}^{2}} \\{x_{2}^{2} + y_{2}^{2}} \\\vdots \\{x_{i}^{2} + y_{i}^{2}} \\\vdots \\{x_{n}^{2} + y_{n}^{2}}\end{bmatrix}}$ A x = b

If there are three points, then the A matrix is square, and the solutionmay be expressed by inverting the A matrix as follows:

x=A⁻¹b

Where more than three points are available, the pseudo inverse may beemployed to provide the least squares solution to the problem as notedabove. This may be states as:

x=(A ^(T) A)⁻¹ A ^(T) b

This solution minimizes the squared error between the circle's perimeterand all the points. From the solution for the vector, x, the centerlocation and estimated radius may be calculated for a circular waferfrom D, E, and F.

For notch detection, the distance of each detected point from thecalculated center may be determined, and any point not conforming to thedesired circularity (using any suitable metric) may be removed, afterwhich the center and radius may be recalculated. An alignment notch canthus be detected in these calculations by identifying detected edgepoints that are off the calculated circle by more than somepredetermined threshold or tolerance. For purposes of center finding,these points may be removed. General information about wafer geometrymay also be employed to detect (and exclude from subsequentcalculations) points that are likely associated with robotic componentsrather than a wafer. In one aspect, the system may discriminate betweenanomalies close to the expected circumference (which are likely due toan alignment notch) and anomalies that are far from the expectedcircumference, so that the rotational alignment of the wafer can also berecovered. In general, such discrimination may be based on the relativemagnitude of the variation, as well as the general notion that analignment notch is characterized by an unexpected absence of a waferwhile the robotic arm would generally cause an unexpected presence of awafer.

In addition, various events during movement, such as radialdisplacement, linear displacement, or other simple or composite motionof a wafer relative to an end effector may be detected and accounted forusing techniques known to one of ordinary skill in the art.

A number of functions related to wafer detection may be usefullyperformed. For example, the system designed herein may calculate linkoffsets for a robotic arm, calibrate sensor locations, calibrate beamwidths for optical sensors, calculate a wafer center position relativeto an end effector, sense wafer presence at predetermined locations,determine when slot valve doors are clear or blocked, and provide foraccurate placement of wafers in process modules, load locks, and otherlinking modules within a fabrication facility. A number of relatedprocessing examples are provided below.

Using the above techniques, as well as any other suitable center findingtechniques, a robotic handler and sensors may be operated to determinewafer location. In one embodiment, the system may track sensor dataduring a retract (step 304) and rotate (step 306), and begin wafercenter calculations upon initiation of extension (step 308). In thisembodiment, after the rotation, a processor may calculate instantaneousradius and angle of the wafer center (using, for example, the leastsquare fit described above), and calculate sensor positions, such as bytransformation to a suitable global coordinate system (e.g., endeffector, module, or the like). This estimated radius may be compared toan expected value, with any anomalies detected and removed. An errorvector may then be derived from these measurements for subsequent sensortransitions and applied to correct prospective trajectory for the wafer.Thus in one aspect a robot handler may gather sensor data during aretract and rotate, and calculate wafer position while gatheringadditional sensor data during an extend.

Other techniques may be employed for center finding calculations. In oneembodiment, a Kalman Filter may be employed using real time encoderupdates (for example, at 2 kHz, every 0.5 milliseconds, every 50milliseconds, or any other suitable frequency or time increments), alongwith time data for each sensor transition event.

FIG. 4 depicts a wafer center finding method employing a Kalman Filter.In general, calculating a wafer position, as shown in step 330 may beperformed using a Kalman Filter that apply encoder data to determinewafer position and/or predict sensor transitions. However, as avariation to the general method depicted in FIG. 3, the (center-finding)Kalman model may be updated periodically. More specifically, sensor datamay be received at each sensor transition that includes a time of thetransition and, as appropriate, an identity and/or location of thesensor, as illustrated in step 330. Based upon this data, an error maybe calculated between an expected transition time for the location andthe measured transition time, as depicted in step 410. This error datamay then be employed to update the Kalman Filter for more accurate,subsequent estimations, as depicted in step 420. Thus in general encoderdata is employed to provide wafer center data for control of the roboticarm, while actual detected transitions may be employed to update thecenter-finding model, for example, the equations of an extended KalmanFilter.

By way of example, for a wafer located at a particular position(X_(e),Y_(e)) and traveling at an estimated velocity and acceleration,V, a. the model might predict a sensor triggering at time t_(e), and thesystem may identify the actual transition at a time t_(s). The encoderpositions measured at that time, t_(s) (or optionally, the time stamp)may generate an error expressed as:

$\delta = {{t_{s} - {t_{e}\mspace{20mu} {or}\mspace{20mu} \underset{\_}{\delta}}} = {\begin{bmatrix}t \\x \\y\end{bmatrix}_{s} - {\begin{bmatrix}t \\x \\y\end{bmatrix}_{e}.}}}$

Then, extended Kalman filter equations may be used as described forexample in Applied Optimal Estimation by Arthur Gelb (MIT Press 1974).An adaptation of the formulation described in Gelb may be briefly statedas a system model:

{dot over (x)}(t)=f(x(t),t)+w(t);w(t)≈N(0,Q(t))

and a measurement model:

Z _(k) =h _(k)(x(t _(k)))+v _(k) ;k=1, 2, . . . v _(k) ≈N(0,R _(k))

with state estimate propagation:

{dot over (x)}(t)=f({circumflex over (x)}(t),t)

and error covariance propagation:

{dot over (P)}(t)=F({circumflex over (x)}(t),t)P(t)+P(t)F^(T)({circumflex over (x)}(t),t)+Q(t)

As a significant advantage, this generalized technique permits use ofindividual sensor events incrementally, rather than requiring somenumber of points (such as three) to identify a circular wafer. It willbe understood that, while a particular order of steps is implied by FIG.4, that the depicted operations are repetitively performed duringoperation of a robotic wafer handler, and that no particular order ortiming of steps should be inferred. Nonetheless, it will be generallytrue in some implementations that encoder data is provided continuouslyin real time, while transitions that initiate model updates would onlyoccur intermittently as a wafer is moved by the robot. It should also beunderstood that, while an extended Kalman Filter is one useful techniquefor converting encoder data into wafer center information, other filtersor linear modeling techniques may similarly be applied.

The methods and systems described above are generally applicable towafer center finding using detection of a wafer at discrete points. Itis also possible to employ a number of linear sensors such a lineararray of charge coupled devices or a contact image sensor to capturewafer data in linear segments. A number of devices employing linearsensors are described below. In these techniques, center finding maygenerally be accomplished through direct analysis of image data, ratherthan inferences drawn from a number of discrete sensor events as withthe techniques described above.

FIG. 5 shows a device with a linear image sensor for capturing imagedata from passing wafers. The device 500 may include a top surface 502,a bottom surface, 504, an interior 506, a linear image sensor 508, alight source 510, and a wafer 512.

The device 500 may be, for example, any device used in a semiconductorfabrication process such as a load lock, buffer, aligner, robotichandler, or the like. In one embodiment, the device 500 is a robotichandler including a robotic arm (not shown) with an end effector forhandling a wafer.

The top surface 502 and bottom surface 504 may partially enclose theinterior 506. Although not depicted, it will be understood that thedevice 500 may also have sides which may, for example, include a numberof entrances for passage of wafers, as well as slot valves or otherisolation mechanisms for isolating the interior 506 of the device 500.In general, the shape and size of the various surfaces of the device 500are not important; however, at least one of the surfaces should beparallel to a plane of movement for wafers so that image sensors can beplaced thereon to capture image data from wafers moving through theinterior 506.

The linear image sensor 508 may be placed on the top surface 502 of thedevice 500 as depicted, or on the bottom surface of the device. In oneembodiment, the linear image sensor 508 may be a contact image sensor. Acommercially available contact image sensor generally includes a lineararray of detectors (such as charge coupled devices) with an integratedfocusing lens and a light source 510, such as LEDs flanked alongside thelinear sensor array. While conventional contact image sensors employred, green, and blue LEDs, or a similar broad spectrum light source,wafers may be suitably imaged for center finding using only a singlecolor source, such as red LEDs. In general, a contact image sensor isplaced in close proximity to an object to be scanned. In otherembodiments, the linear image sensor 508 includes a linear array ofcharge coupled devices (“CCDs” or complementary metal oxidesemiconductor (“CMOS”) optical sensors. The linear array may be a 1-by-narray that includes n sensors (such as 128 sensors, or any othersuitable number for spanning some or all of a wafer), a 2-by-n array, orany other suitable one dimensional or two dimensional array. In general,CCDs or CMOS devices may be placed further from an object being imagedand provide greater resolution than current CIS devices. However, theyrequire additional external lighting for good image capture quality. Onthe other hand, CIS devices are readily available in lengths exceedingthe diameter of typical semiconductor wafers, provide an inexpensivealternative for image capture, and provide greater accuracy forpre-packaged arrays. While both technologies are suitable for use withthe embodiments described herein with suitable adaptations for someapplications, each offers advantages which may make it more suitable forcertain uses. Some of these variations are described below, however, asnoted above either of these technologies, or other optical technologies,may be usefully employed as linear image sensors 508 as that term isused herein. The linear image sensor 508 has a field of view ormeasurement volume in which image data may be captured. In general, thelinear image sensor 508 may have an operative measurement volume thatdepends on a number of factors including ambient light, desired accuracyof image, lenses or other optics associated with the sensor, and soforth.

The wafer 512 may be passed through the device 500 in a linear path asindicated by an arrow 514. It will be understood that while a linearpath is one possible motion for a wafer, many other motions may beapplied by a robotic handler. For example, the wafer may move in acurving path with a rotational movement of a robot, or may move in adiscontinuous path formed of a number of different linear and/or curvingpaths. As will be further discussed below, the wafer may also or insteadbe rotated about its axis. It will be appreciated that, while the dataobtained from such scans can generally be directly analyzed to locate awafer center and obtain other wafer position data (such as rotationalorientation, radius, etc.), that the acquired image data must becoordinated with robot motion using, for example, encoder data or othersensor data, in order to correctly interpret the image data.

FIG. 6 shows a top view of a contact image sensor used for wafer centerfinding with a linear wafer motion. Within a device, which may be any ofthe devices 500 described above, a wafer 602 having an alignment notch604 may be passed in a linear motion (denoted by an arrow 606) by asingle CIS 608 positioned normal to the direction of motion 606. Inembodiments, the CIS 608 may include a single module having a length of310 millimeters, and may be positioned across an entrance to the deviceto provide full wafer detection, include notch/alignment detection, asthe wafer is moved into and out of the device through the entrance. Thistype of wafer detection provides, in effect, a photocopy of the wafer602 from which alignment and dimensions may be directly obtained byimage analysis. As a significant advantage, this arrangement provides afull wafer scan without requiring any additional robot arm movements orthe like. Thus throughput for the transfer device may proceed at a speedthat is limited only by robotic and other constraints. In otherembodiments, one such CIS 608 may be placed at each of severalentrances, for example, at four entrances of a square robotic handler. Asingle CIS 608 may also, or instead, be positioned to intersect a centerof the device. Using a CIS 608 of approximately 450 millimeters, asingle CIS may be positioned at a forty-five degree angle to all fourentrances and intersecting a center of the device to permit capture ofall linear wafer movements through the device. While this arrangementmay not capture all wafer dimensional data for all movements through thedevice, it may nevertheless provide sufficient data for wafer centerfinding for any possible movement, and additional movements may beprovided by a robotic handler to ensure a scan of the entire wafersurface.

FIG. 7 shows a top view of a contact image sensor used for wafer centerfinding with a curving wafer motion. Within a device, which may be anyof the devices 500 described above, a wafer 702 having an alignmentnotch 704 may be passed in a curving motion (denoted by an arrow 706)across a single CIS 708. This arrangement may be suitable forpositioning at a variety of locations within a robotic handler where therobotic arm employs rotation, although it will be understood that theresulting image data would typically be processed to compensate for thenon-linear path 706 taken by the wafer 702.

FIG. 8 shows a top view of a contact image sensor used for wafer centerfinding with a rotating wafer motion. Within a device, which may be anyof the devices 500 described above, a wafer 802 having an alignmentnotch 804 may be rotated about an axis substantially centered on a CIS808, as indicated by an arrow 810. In this device, a robotic handler mayinclude z-axis control and a rotating chuck. The robotic handler mayposition the wafer 802 underneath and centered on the CIS 808, and thenoptionally lift the wafer 802 into closer proximity to the CIS 808 formore accurate image acquisition. The wafer may then be rotated onehundred and eighty degrees (or more) to obtain a complete image of thewafer 802 including the alignment notch 804. The CIS 808 may be centeredwithin the device, such as at a central axis of an interior of thedevice, a center axis of a robotic arm inside the device, or a centeraxis of some other robotic home position within the device. Thisarrangement advantageously obtains a full scan with a half-turn of therotating chuck, which may simplify design of the chuck and reducescanning time. As another advantage, this arrangement can provide a fullwafer scan regardless of wafer size (within a limit imposed by thelength of the CIS 808). Thus a single system may provide full edgedetection for a variety of shapes and sizes.

FIG. 9 shows a pair of linear CCD arrays used for wafer center findingwith a linear wafer motion, which may be deployed, for example at anentrance to a device such as any of the devices 500 described above. Inthis embodiment, a first linear array 902 and a second linear array 904of CCDs may be provided across a portion of a linear path 906 of a wafer908. The arrays 920, 904 may be positioned, for example, along theoutside edges of an entrance to the device such as a robotic handler inorder to capture image data for each wafer passing through the entrance.Similarly, an additional pair of sensor arrays may be positioned at oneor more additional entrances to the device. While this configurationadvantageously employs short linear arrays of CCDs which are readilycommercially available, it also may fail to capture an alignment notchused to determine rotational alignment of the wafer 908.

FIG. 10 shows a single CCD array used for wafer center finding with arotating wafer motion. In this embodiment, a single linear CCD array1002 may be positioned on a lid or other suitable interior surface of adevice such as a robotic handler or any of the other devices 500described above. After a wafer 1004 is suitably positioned under thearray 1002, the wafer 1004 may undergo a full rotation as indicated byarrow 1006 so as to capture all edge data for the wafer 1004 includingthe position of an alignment notch 1008. This embodiment may, forexample, employ a robotic handler with z-axis movement and a rotatingchuck as described above. However, in this embodiment the rotating chuckpreferably rotates at least three-hundred sixty degrees to ensure fullcapture of edge data. In other embodiments, two collinear arrays may beemployed at opposing edges of the wafer 1004 in order to obtain a fulledge scan with a half rotation.

FIG. 11 shows four CCD arrays used for wafer center finding with acompound wafer motion. As depicted, a device such as any of the devices500 described above may include four CCD arrays 1102 arranged in twocollinear, intersecting lines, to cover wafer paths in a mannersubstantially similar to that discussed above with reference to FIG. 1.A wafer 1104 may traverse an interior of the device along a path 1106that includes straight and curving motions. In one embodiment, the wafer1104 may be retracted sufficiently toward the center to ensure detectionof an alignment notch 1108 at some point during the combined motion ofthe wafer 11104.

FIG. 12 shows a top view of CCD sensors on a robotic arm end effector. Arobotic arm 1200 for wafer handling may include a number of links 1202and an end effector 1204. The end effector 1204 may include a number oflinear CCD arrays 1206 positioned, for example to identify four edgelocations of a wafer 1208 positioned thereon. As a significantadvantage, this configuration places the wafer 1208 in very closeproximity to the linear CCD arrays 1207, which provides very high imageaccuracy. Further, this design does not require any z-axis or rotationalmotion by the end effector 1204. It will be apparent from FIG. 12,however, that this configuration may also fail to identify an alignmentnotch for many rotational orientations of the wafer 1208.

FIG. 13 shows a perspective view of single CCD sensor on an end effectorwith a rotating chuck. In this embodiment, a single linear CCD array1302 may be mounted on an end effector 1304 at a position to obtain edgedata from a wafer 1306 substantially centered on the end effector 1304.The end effector may also include a single axis rotating chuck to rotatethe wafer 1306 in a full circle in order to obtain complete edge datafrom the wafer 1306, including detection of an alignment notch, if any.

A number of external devices 1320 may support use of the CCD array 1302.For example, an external light source may be positioned within thedevice to illuminate the CCD array 1302 while the end effector 1304 isin certain positions. As another example, a power source may be providedthat is inductively coupled to the CCD array 1302 so that the CCD array1302 is wirelessly powered within the vacuum environment. As anotherexample, a radio frequency or other wireless transceiver may be employedto receive image data wirelessly from the CCD. In such wirelessconfigurations, transceivers, power couplings and the like may bepositioned away from the CCD array, such as at a center axis of therobotic arm or some other location that is closer to correspondingwireless systems.

FIG. 14 shows a single CCD sensor in a robotic handling module. In thisembodiment, a single linear CCD array 1402 and any associated lightsources or other emitters may be mounted on an interior wall of a devicesuch as a robotic handler or any of the other devices 500 describedabove. In operation, an end effector 1404 may position a wafer 1406 sothat the wafer 1406 is centered on a rotating chuck 1408 (separate fromthe end effector 1404) with an edge above the CCD array 1402. The endeffector 1404 may then provide an z-axis motion as indicated by an arrow1410 to lower the wafer 1406 onto the chuck 1408. The chuck 1408 maythen rotate the wafer 1406 in a complete revolution to provide a scan ofthe entire wafer perimeter. In addition to capturing position data forthe wafer 1406, this approach captures rotational orientation of thewafer 1406 by detecting an alignment notch, if any, on the wafer 1406.As in the embodiment of FIG. 13, a device 1420 such as a light source,wireless power coupling, or wireless data transceiver may be positionedwithin the interior, or where appropriate, on the exterior of the moduleto enhance operation of the wafer center finding systems describedherein.

It will be understood that, while the embodiments described aboveinclude sensors within a device such as a load lock, robotic handler, ortransfer station (or in certain embodiments, on an end effector), thatthe above techniques may be deployed at other locations within afabrication system. For example, any of the above techniques may besuitably adapted for use as an aligner. Similarly, a number of the abovetechniques may be suitably adapted for use as a measurement stationwithin another device, such as a robotic handler or transfer station. Insuch embodiments, the measurement station may scan a wafer while a robotperforms other wafer movements, such as by providing a space for themeasurement station that does not obstruct other entrance or exit pathsfrom the robotic handler, or by performing measurements at a locationdisplaced on the z-axis from other robotic activities.

It will be appreciated that the methods disclosed herein may be realizedin hardware, software, any some combination of these suitable formonitoring or controlling a semiconductor manufacturing robotics system.Each process may be realized in one or more microprocessors,microcontrollers, embedded microcontrollers, programmable digital signalprocessors or other programmable device(s), along with internal and/orexternal memory. The process(es) may also, or instead, be embodied in anapplication specific integrated circuit, a programmable gate array,programmable array logic, or any other device or combination of devicesthat may be configured to process electronic signals. It will further beappreciated that process(es) may be realized as computer executable codecreated using a structured programming language such as C, an objectoriented programming language such as C++, or any other high-level orlow-level programming language (including database programming languagesand technologies) that may be compiled or interpreted to run on one ofthe above devices, as well as heterogeneous combinations of processors,processor architectures, or combinations of different hardware andsoftware. All such variations are intended to fall within the scope ofthis disclosure.

While the invention has been described in connection with certainpreferred embodiments, other embodiments may be understood by those ofordinary skill in the art and are encompassed herein.

1. A method for finding a center of a wafer in a device having aninterior and a plurality of entrances, the interior containing a roboticarm, and the device including a plurality of sensors, each one of thesensors adapted to detect a presence of the wafer at a predeterminedlocation within the interior of the device, the method comprising:retrieving the wafer from outside the interior through a first one ofthe plurality of entrances; retracting the wafer into the interior anddetecting a presence of the wafer with a first one of the plurality ofsensors; rotating the robotic arm; extending the wafer out of theinterior through a second one of the plurality of entrances anddetecting an absence of the wafer with the first one of the plurality ofsensors; and determining a location of a center of the wafer relative tothe robotic arm using sensor data from the plurality of sensors andposition data from the robotic arm.
 2. The method of claim 1 wherein theplurality of sensors include optical sensors.
 3. The method of claim 2wherein the plurality of sensors include light emitting diodes.
 4. Themethod of claim 2 wherein the plurality of sensors include auto focusingphotodiode detectors.
 5. The method of claim 1 wherein determining alocation includes applying a Kalman Filter to the position data from therobotic arm.
 6. The method of claim 5 further comprising updating theKalman Filter based upon the sensor data.
 7. The method of claim 1wherein the wafer is substantially circular.
 8. The method of claim 1wherein the wafer includes an alignment notch.
 9. The method of claim 1wherein the plurality of sensors include at least one detectorpositioned opposite a light emitting diode such that a light path fromthe light emitting diode to the light detector includes a predeterminedposition within the interior.
 10. The method of claim 1 wherein theplurality of sensors include at least one detector positioned such thatlight from a light emitting diode, when reflected off of the wafer at apredetermined location, is detected by the detector.
 11. The method ofclaim 1 wherein retracting includes retracting in a linear motion. 12.The method of claim 1 wherein extending includes extending in a linearmotion.
 13. The method of claim 1 wherein rotating includes rotatingabout a center axis of the robotic arm.
 14. A method for finding acenter of a wafer in a device having an interior and a plurality ofentrances, the interior containing a robotic arm, and the deviceincluding a plurality of sensors, each one of the sensors adapted todetect a presence of the wafer at a predetermined location within theinterior of the device, the method comprising: retrieving the wafer fromoutside the interior through a first one of the plurality of entrances;retracting the wafer into the interior; rotating the robotic arm;extending the wafer out of the interior through a second one of theplurality of entrances; detecting the presence of the wafer at apredetermined location of at least one sensor during the retracting,rotating, and extending, thereby providing sensor data; and determininga location of a center of the wafer relative to the robotic arm usingthe sensor data and position data from the robotic arm.
 15. The methodof claim 14 wherein the plurality of sensors include optical sensors.16. The method of claim 15 wherein the plurality of sensors includelight emitting diodes.
 17. The method of claim 15 wherein the pluralityof sensors include auto focusing photodiode detectors.
 18. The method ofclaim 14 wherein determining a location includes applying a KalmanFilter to the position data from the robotic arm.
 19. The method ofclaim 18 further comprising updating the Kalman Filter based upon thesensor data.
 20. The method of claim 14 wherein the wafer issubstantially circular.